In a once-through steam generator the heating of a number of steam generator tubes which together form an evaporator heating surface leads to a complete evaporation of a flow medium in the steam generator tubes in one pass. The flow medium—usually water—is generally fed before its evaporation to a preheater, usually also referred to as an economizer, connected upstream on the flow medium side from the evaporator heating surface and preheated there.
As a function of the operating state of the once-through steam generator and associated therewith as a function of the current steam generator output the feed water mass flow in the evaporator heating surface is regulated. With changes in load the evaporator flow should be changed as synchronously as possible with the input of heat into the evaporator heating surface, because otherwise a deviation of the specific enthalpy of the flow medium at the exit of the evaporator heating surface from the target value cannot be securely avoided. Such an unwanted deviation of the specific enthalpy makes the regulation of the temperature of the fresh steam exiting from the steam generator and also leads to high material stresses and thus to a reduced lifetime of the steam generator.
To keep deviations of the specific enthalpy from the target value and the resulting unwanted large temperature fluctuations in all operating states of the steam generator, i.e. especially also in transient states or during changes in load, as low as possible, the supply water flow regulation can be embodied as a type of a so-called predictive design. In such cases the necessary supply water target values, especially also during a change of load, should be provided as a function of the current operating state or of the state to be expected in the near future.
A once-through steam generator is known from EP 0639 253 in which the supply water flow is regulated using a predictive calculation of the necessary amount of supply water. The calculation method is based in this case on the heat flow balance of the evaporator heating surface in which the supply water mass flow, especially at the entry of the evaporator heating surface, should be included. The target value for the supply water mass flow is predetermined on the one hand from the ratio of the heat flow transferred in the evaporator heating surface to the flow medium and on the other hand from a target enthalpy increase of the flow medium in the evaporator heating surface predetermined in respect of the desired live steam state.
In practice the measurement of the supply water mass flow directly at the entry of the evaporator heating surface however proves technically complex and is not able to be carried out reliably in each operating state. Despite this the supply water mass flow is measured instead at the entry to the economizer and included in the calculations of the supply water mass flow at the entry of the evaporator heating surface.
To counter the imprecisions caused by this in the predetermination of an especially demand-related target value especially during changes in load for the supply water mass flow, in an alternate concept of a predictive mass flow regulation, as is known in WO 2006/005708 A1, there is provision to take into consideration the supply water density at the entry of the economizer as one of the input variables for the supply water flow regulation.
Both said concepts for a predictive mass flow regulation are based as a major input variable on the target value for the steam generator power, from which on the basis of stored correlations and especially referring back to previously obtained calibration or reference measurements, the characteristic values included in the actual target flow value determination are calculated. This however requires system characteristics which are sufficiently stable and able to be referred back to a firing power, as are usually present with fired steam generators. In other systems, such as when the once-through steam generator is designed as a waste-heat boiler for heat recovery from the flue gas of an upstream gas turbine for example, these types of conditions are not available. In addition, with such systems connected as waste-heat boilers, a firing power is not usable to the same degree as a free parameter as with directly-fired boilers, since with a connection as waste-heat boilers the operation of the gas turbine is usually seen as the primary criterion for controlling the overall system, to the system state of which the other components are adapted.